Add support for stage instances

Created by: IDoEverything

Summary

Adds support for stage instances.

Details

Added the entities, endpoints, events and some caching for stage instances.

Changes proposed

• Add DiscordStageInstance and PrivacyLevel
• Add stage instance endpoints to DiscordClient and DiscordRestClient
• Add stage instance events
• Add StageInstances to DiscordGuild and properly cache them on the events.